People are generating an ever-increasing number of documents. As the number of documents grows, it has become increasing difficult for people to find a relevant document amid the plethora of available documents. Consequently, search systems that identify relevant documents have been increasingly important. However, it is insufficient for a search system to merely identify relevant documents: the search system should identify relevant documents in a timely manner. To improve the efficiency of a search system, the search system needs to collect information regarding the performance of the search system.